Method of making fuel cell electrolyte matrix

ABSTRACT

A heat resistant porous fuel cell matrix for holding liquid electrolyte in fuel cells which can be subjected to extreme temperature conditions without cracking and permanently deforming is made from a ceramic metal oxide such as magnesia, a binder such as an alkali metal phosphate, and a liquid phase sintering agent for the metal oxides such as lithium fluoride. To form the matrix, the matrix molding composition is formed containing a thoroughly mixed composition of a major portion of the ceramic metal oxide, and minor portions of the binder and liquid phase sintering agent. The composition is molded into suitable matrix form, and the binding agent is activated such as by heating to form a porous green matrix which has structural integrity at room temperature. The porous green matrix is then heated to a temperature above the melting point of the liquid phase sintering agent to cause substantial sintering between the metallic oxide particles and yield a thermal resistant porous fuel cell matrix.

[ 1 Apr. 25, 1972 [54] METHOD OF MAKING FUEL CELL ELECTROLYTE MATRIX[72] Inventor: Foster L. Gray, Dallas, Tex.

[73] Assignee: Texas Instrument Incorporated, Dallas,

Tex.

[22] Filed: Mar. 13, 1969 [21] Appl.No.: 806,901

Primary ExaminerDonald L. Walton Attorney-Samuel M. Mims, Jr., James 0.Dixon, Andrew M. Hassell, Harold Levine, Melvin Sharp and John M.Harrison 57 ABSTRACT A heat resistant porous fuel cell matrix forholding liquid electrolyte in fuel cells which can be subjected toextreme temperature conditions without cracking and permanentlydeforming is made from a ceramic metal oxide such as magnesia, a bindersuch as an alkali metal phosphate, and a liquid phase sintering agentfor the metal oxides such as lithium fluoride. To form the matrix, thematrix molding composition is formed containing a thoroughly mixedcomposition of a major portion of the ceramic metal oxide, and minorportions of the binder and liquid phase sintering agent. The compositionis molded into suitable matrix form, and the binding agent is activatedsuch as by heating to form a porous green matrix which has structuralintegrity at room temperature. The porous green matrix is then heated toa temperature above the melting point of the liquid phase sinteringagent to cause substantial sintering between the metallic oxideparticles and yield a thermal resistant porous fuel cell matrix.

6 Claims, 3 Drawing Figures METHOD OF MAKING FUEL CELL ELECTROLYTEMATRIX This invention relates to fuel cells in another aspect, thisinvention relates to an improved matrix for a fuel cell which will notcrack or permanently distort when subjected to temperature extremes.

A conventional fuel cell configuration includes at least a pair ofporous, conductive electrodes (at least one cathode and one anode)spaced apart and contacted by an electrolyte which is carried by amatrix of dielectric material, which provides a multiplicity of pores.In the operation of this cell, a suitable reactant gas is passed to eachelectrode and contacts electrolyte in the porous structure of eachelectrode to provide for cell reaction. Thus, at each electrode, a halfcell chemical reaction occurs: Between reductant and electrolyte at oneelectrode and between oxidizer and electrolyte at the other electrode.These reactions create an electric potential between the electrodes, andthereby furnish electric power to an external circuit.

The present trend in fuel cell development is toward lighter, smaller,and thinner fuel cells. The trend has resulted in the development ofclosely spaced anodes and cathodes, and therefore, in very thin butuniform electrolyte containing matrices. It is generally necessary thatthe thin matrices utilized in the cells be uniform in thickness andporosity so that a substantially uniform contact of electrolyte is madewith the surface of the electrodes.

One method of forming the thin matrices is initially to form a moldingcomposition of matrix material and then apply this material uniformly onthe surface of an electrode, for example, the anode. The cathode is thenplaced against the exposed surface of the matrix to form a fuel cellunit. The electrolyte can be added to the matrix by depositingelectrolyte on the exposed matrix area between the electrodes. Theelectrolyte will then wick through and saturate the thin matrix betweenthe electrodes.

While the older conventional matrix material comprises a multiplicity offinely divided ceramic material such as magnesium oxide particles,aluminum oxide particles, or lithium aluminate particles, the recentlydeveloped thin wall matrix material includes, in addition to the ceramicparticles, binders such as silicates, phosphates, borates, andaluminates. These binders are generally necessary to give the thinmatrices structural integrity (1 at room temperatures to facilitate fuelcell assembly, and (2) at the higher operating temperatures of the fuelcell.

Problems have occurred when using the newly developed thin matrices,particularly in conjunction with molten carbonate electrolyte fuelcells. It has been found, for example, that the molten carbonateelectrolyte will react with some binder materials such as the silicatesto form reaction products which in turn plug the pores of the matrix.This results in nonuniform contact of the electrolyte on the electrodes.Additionally, reaction of the electrolyte with most of theseconventional binder materials will weaken and/or destroy their abilityto bind the ceramic particles together, and prolonged high temperatureusage will thereby yield the matrix soft during the fuel cell operation.This results in non-uniform contact between the matrix and theelectrodes, especially in the applications where the matrix material ismolded on a very rough or wavy surface of an electrode such as an anode.

In addition to the above described problems it has been found that thesenewly developed thin matrices which are bound together by theconventional binder materials will crack and/or permanently distort whensubjected to extremely high temperatures, for example, 900 C. Morespecifically, in my copending application Ser. No. 806,838, filed onMar. 13, 1969, there is disclosed a method of connecting electrodes inseries by a brazing technique which generally requires fully assembledanodes carrying thin matrices to be assembled with cathodes by brazingtechniques which subject the assembled anode, matrix, and cathodes to atemperature of at least 900 C. The thin matrices which are bound by theconventional binders crack and sometimes permanently distort because ofthis thermal cycling.

Therefore, one object of this invention is to provide improved matrixfor fuel cells and method for making the same.

Another object of this invention is to provide a matrix for fuel cellswhich will maintain its structural integrity when sub- 5 jected toextreme thermal cycling prior to the time that it has been impregnatedwith electrolyte.

A further object of this invention is to provide an improved fuel cellmatrix which will maintain its structural integrity after it has beenoperated for extended periods of time in a fuel cell when impregnatedwith electrolyte.

According to one embodiment of this invention, a porous ceramic fuelmatrix is provided which consists of ceramic metal oxide particles heldtogether at least partially by selfbonds induced by liquid phasesintering and at least partially by a dissimilar binding agent.

According to another embodiment of this invention, a process of makingthe above described matrix is provided whereby matrix molding mixture isinitially formed which consists of a major effective portion of aceramic metal oxide, a minor effective portion of a binding agent forconsolidating the ceramic particles into a porous mass, and a minoreffective portion of a liquid phase sintering agent for the ceramicmetal oxide particles; the molding mixture is formed into the desiredmatrix shape and the binder agent is activated to hold the particles ina consolidated relationship in the molded shapes; and lastly, the moldedporous matrix is heated to a sintering temperature of the ceramic metaloxide particles above the melting point of the liquid phase sinteringagent, and the ceramic metal particles are thereby at least partiallyself-bonded.

This invention can be more easily understood from a study of thedrawings in which:

FIG. 1 is a perspective view of a conventional fuel cell containing theimproved matrices of this invention;

FIG. 2 is a sectional view along lines 2-2 of FIG. 1; and

FIG. 3 is a perspective view of a matrix of this invention which ismolded onto an electrode.

Now referring to FIGS. 1 and 2, fuel cell unit 10 generally comprises amulticell power package incorporating fuel cells interconnected inseries, and in parallel. FIGS. 1 and 2 are given for illustrativepurposes only to illustrate the improved matrix of this invention. Thus,the number of fuel cells in Unit 10 together with the electricalinterconnections therebetween is not intended to limit the scope of thisinvention.

Enclosures 11 and 12 which carry cavities I3 and 14, respectively, areconnected at opposite ends of fuel cell 10. Fuel inlet conduit 15communicates through the enclosure 11 and has annular electric terminal16 operatively attached thereto. In similar manner, fuel outlet conduit17 communicates through enclosure 12 and has annular electric terminal18 operatively connected thereto. Electrical conductive wires lead fromterminal 16 and terminal 18 to a suitable circuit.

Now referring to FIGS. 2 and 3, electrodes 19 and 190 are identical andgenerally comprise a single sheet metal screen formed into a series ofteardrop-shaped folds. The teardropshaped folds are held together bybrazing or spot welding points of contact such as illustrated at points20. This folded pattern allows reactive fuel to freely flow throughspaces 21. Alternately a variety of other electrode structures can beutilized for electrodes 19 and 19a. For example, a coarse wire meshsimilar to a kitchen scour pad can be used, or a number of cylindricalor tubular-shaped pieces stacked one on another can be used. Electrodes19 and 19a can be made from a suitable anode material. For example, -100mesh nickel screen.

Electrodes 19 and 19a carry matrices 22 and 22a respectively, moldedaround the outside periphery thereof as illustrated in FIG. 3. Matrices22 and 22a are molded by the process of this invention which willdescribed in detail below. These matrices are porous, crack-free ceramicbodies having high structural integrity and resistance to cracking andpermanent distortion when subjected to temperature extremes.

Electrodes 23 and 23a are identical and each generally comprise two sidesheet members held apart by two end channels, and a corrogated memberwelded therebetween. Electrodes 23 and 23a serve as the cathodes forfuel cell unit 10 and can be made of any suitable porous material knownin the art such as 80-150 mesh silver plated stainless steel meshscreen, for example.

The electrical connections between the electrodes and electricalterminals 16 and 18 are illustrated in FIG. 2. Conductive end plate 24is connected such as by welding with conductive enclosure 11. Slot 25through conductive plate 24 communicates from chamber 13 to spaces 21within the electrodes 19 and 19a. Likewise, conductive end plate 26 isconnected such as by welding to conductive enclosure 12 and therebycommunicates with electric terminal 18. Slot 27 communicates betweenchamber 14 and spaces 21 of electrodes 19a and 19. Electrodes 23 areoperatively connected to conductive end plate 24 at points 28 whileelectrode 19 is spaced from and thereby insulated from conductive endplate 24. Electrode 19a is operatively connected to conductive end plate26 while electrodes 23a are spaced and insulated from conductive endplate 26. Electrode 19 is operatively connected to electrodes 23a byflanges 29. In this arrangement electrodes 19 and 19a serve as dualanodes for two cells respectively. Additionally, electrodes 23 areconnected in parallel, and electrodes 23a are connected in parallel bychannels 30 and 30a, respectively (FIG. 1). As illustrated in FIG. 1,channels 30 and 30a are positioned over the top and the bottom ofmatrices 22 and 22a respectively, and electrodes 23 are operativelyconnected to channel 30 while electrodes 23a are operatively connectedto channel 30a.

Fuel cell unit will operate in any conventional manner. This particularsystem can effectively use various alkali metal carbonates aselectrolytes. A preferred electrolyte is a eutectic mixture of sodiumcarbonate and lithium carbonate, e.g. 50 percent molar sodium carbonateand 50 percent molar lithium carbonate having a melting point of about500 C. The matrices are initially impregnated with the electrolyte. Fuelcell unit 10 is placed within a suitable environment wherein an oxidizerreactant will continuously pass through electrodes 23 and 23a.Electrodes l9 and 19a are provided with a suitable fuel via inletconduit and chamber 13. Fuel cell unit 10 will function with a varietyof reactants, but the preferred system is a fuel feed comprisinghydrogen and oxidizer mixture comprising oxygen and carbon dioxide. Thehydrogen can either be pure or mixed along with various other gases suchas nitrogen, carbon dioxide, carbon monoxide, light hydrocarbons, watervapor, and the like. The oxygen can be either pure or supplied as air.Thus, fuel cell unit 10 is placed within a suitable heating device whichis supplied with the above described oxidizer atmosphere and maintainedat a temperature in the vicinity of 600 C., e.g. about 650 to 700 C. Forexample, placing fuel cell unit 10 within a ceramic wall oven (or otherinsulated casing means) which is provided with a gaseous flow of oxygenand carbon dioxide, or air in carbon dioxide in a direction parallel toelectrodes 23 and 23a will suffice.

The hydrogen gas is then passed through inlet conduit 15, into chamber13 and through slot 25, spaces 21 of electrodes 19 and 19a to chamber14. The reaction occurring in electrodes 19 and 19a (the anodes) is asfollows:

The oxygen and carbon dioxide which passes through the electrodes 23 and23a and surrounds fuel cell unit 10 within the heating device will reactas follows when contact is made with electrodes 23 and 23a:

Conventional matrix materials made from particulate ceramic metal oxidessuch as magnesium oxide, aluminum oxide, and lithium aluminates andbound together by binders such as silicates, phosphates, aluminates, andfluorates, generally lose their structural integrity at least in partduring prolonged periods of contact with the molten carbonateelectrolyte at the elevated temperatures of fuel cell operation. Thiscan result in non-uniform contact of an electrode by the electrolyte andineffecient cell operation. Additionally, it has been found that if thematrix is thermal cycled in the range of about 900 C., the conventionalmatrices will crack and permanently distort. This thermal cycling occurswhen practicing the process of my copending application, Ser. No.806,838 filed Mar. 13, 1969 which discloses improved process for brazingfuel cell modules such as illustrated in the drawing by initiallyassembling the modules including the anodes carrying a matrix moldedthereon by means of spot welding, applying a brazing material betweenthe interconnected component parts and heating the temperature to asuitable brazing temperature, for example a temperature above 900 C. Theconventional matrix material molded to the anodes in the assembliesgenerally will not stand the thermal cycling and will thereby crack orpermanently distort.

The improved matrix of this invention on the other hand. will withstandprolonged conditions of high temperature fuel cell operation withoutsubstantial softening and deteriorating, and will withstand the thermalcycling operations such as described above without cracking orpermanently distorting. The improved matrix of this invention is madefrom a major effective portion of a ceramic metal oxide, a minoreffective portion of a conventional binder material, and a minoreffective portion of a liquid phase sintering agent for the ceramicmetal oxide. The particulate ceramic material, binder, and liquid phasesintering agent utilized in this invention can depend on such factors asthe type of electrolyte, and the fuel cell operating conditions.

Suitable ceramic metal oxides include magnesium oxide, zirconium oxide,and lithium aluminate. From molten carbonate fuel cell operation themagnesium oxide is preferred. Suitable binders include phosphates,silicates, borates and aluminates. From molten carbonate fuel celloperation phosphate binders are preferred since they are substantiallynot reactive with the electrolyte. Particularly preferred for thisapplication are the alkali metal phosphates. For example, sodiumorthophosphate (NaH PO NaHPO sodium pyrophosphate (Na P O), sodiumtripolyphosphate (Na P O sequestered phosphate (glass) [(NaPO where x 68], sodium hexametaphosphate [(NaPO where x 12 l4], and the like.

Any liquid phase sintering agent for the ceramic metal oxide can be usedin the practice of this invention. The liquid phase sintering agent canyield either a reactive or non-reactive solid-liquid system. Frommagnesium oxide the preferred liquid phase sintering agents includelithium carbonate, lithium fluoride, lithium chloride, lithium bromide,lithium iodide and lithium sulphate.

To form the matrix of this invention, the three basic components areinitially thoroughly admixed, generally in the presence of a smallamount of a non-deleterious wetting agent such as water. The threecomponents are generally in a powdered or particulate state. Theparticle size of ceramic metal oxide can vary according to theparticular operation, but generally ranges from about 50 to about 400mesh (U.S. standard). More preferably, the mesh size ranges from about100 to about 325 (U.S. standard). Typical formulations of the ceramicmetal oxide include from 30 to 50 weight percent in the l00 to 200 meshrange, and from 30 to 50 weight percent being smaller than 200 mesh. Thebinder and liquid phase sintering agent are generally in a particulateform sufficient to admix thoroughly with ceramic metal oxide particles.

The ceramic metal oxide generally comprises from about to about 98weight percent, preferably from to 94 weight percent, on a dry weightbasis, of the molding mixture. The binder and the liquid phase sinteringagent each comprise from about 1 to about 10, preferably from about 2 toabout 5 weight percent, on a dry weight basis, respectively, of themolding mixture. These compounds are generally thoroughly admixed withabout 1 to 10 parts by weight of water or other non-deleterious wettingagent. It is generally preferred to initially thoroughly admix theceramic metal oxide with liquid phase sintering agent in a dry state andthen add the binder together with the water to form the moldingcomposition. The composition is then molded into a suitable form such asthat illustrated in FIG. 3 by conventional techniques. Any suitablematrix shape can be formed for any particular application, for example,a rectilinear shape molded on one side of an electrode.

Next the molded matrix is dried and cured for a sufficient time toremove the water therefrom and activate the binder.

,For most mixtures it is sufficient to cure the molded matrix from 1 toabout hours in an air atmosphere at a temperature in the range fromabout 80 to about 150 C.

After the above described curing step, a porous green mold is formedhaving substantial structural integrity at room temperatures. Thus, themolded matrix can be manipulated during fuel cell assembly or the likewithout the danger of distorting, softening or cracking. At this stage,the matrix is heated to a temperature well above the melting point ofthe liquid phase sintering agent (generally from 850 to l,OOO C.) toinduce substantial sintering between the ceramic metal oxide particles.It is preferred to conduct this operation in the presence of anon-oxidizing atmosphere such as H to prevent corrosion of the electrodematerial. The phenomena of liquid phase sintering is described inChapter 16 of the book Ceramic Fabrication Processes, TechnologicalPress of Massachusetts Institute of Technology and the John Wiley &Sons, lnc., New York, London (1958). This sintering process results in astronger matrix than heretofore known in the art which will maintain itsstructural integrity under the above described thermal cyclingconditions and under prolonged high temperature fuel cell operationconditions in the presence of reactive electrolyte.

The actual porosity of the particulate matrix can be varied as desiredby such means as the size for the ceramic oxide particles. For example,consider the equation: y one-half hgdr, where 'y surface tension of aliquid; h the height of the column of the liquid above the lower liquidlevel; g acceleration due to gravity; d density of the liquid; and rradius of the capillary pore. By rearrangement of the equation, it canbe seen that the capillary pore radius is directly proportional to thesurface tension of the liquid and inversely proportional to the heightof the column, the gravitational acceleration and the liquid density.Consequently, with proper sizing of the ceramic particles, the desiredamount of capillarity can be obtained for a given electrolyte.

The following example is given for illustrative purposes to betterfacilitate the understanding of this invention:

EXAMPLE A molding premix was initially obtained by thoroughly admixing33.95 parts by weight of particulate magnesium oxide having a particlesize of less than 200 mesh (U.S. standard), 14.55 parts by weight ofparticulate magnesium oxide having a particle size in the range of 100to 200 mesh (U.S. standard), and 1.5 parts by weight of particulatelithium carbonate. To this molding premix was added 3 parts by weight ofsequestered phosphate and 5.75 parts by weight of water to form amolding paste mixture.

Portions of this paste were molded to one side of an electrode such asillustrated in FIGS. 2 and 3, to form a uniform coat of about onethirty-second inch, and was aged an hour in air at room temperature.Next, the mixture was molded on the other side of the electrode to forma uniform coat of about one thirty-second inch, and the compositions onthe two sides of the electrode were allowed to dry for an additional 3hours at room temperature. After this, the molded matrix was oven dried.Side 1 was oven dried at C. for 2 hours and Side 2 was oven dried at 85C. for 1 hour. Next, both sides were oven dried at 120 C. for 16 hours.

After this initial curing operation, the matrix was dry and phosphatebonded sufficiently to give it good structural integrity at roomtemperature. The electrode carrying the matrix was then heated forminutes at 900 C. in a hydrogen atmosphere. After this 90 minute period,the matrix was allowed to cool. No cracks were present. Additionally,the matrix surface on each side of the electrode was scratched.

The matrix was found to be very strongly bonded together. This matrixperformed well under con mom of fuel cell operation while impregnatedwith molten lithium carbonate electrolyte and maintained at 600 C. forseveral hours. After this time the matrix had not substantially softenedand maintained excellent structural integrity and contact on theelectrode to which it was molded.

While this invention has been described in reference to its preferredembodiments, it is to be understood that various modifications withinthis scope of the appended claims will not be apparent to one skilled inthe art upon reading the specifications.

I claim:

1, A method of making a thermally stable, crack-resistant, porous matrixfor holding electrolyte in a fuel cell comprising:

forming a mixture comprising a major elTective portion of a particulateceramic oxide, a minor effective portion of a binder for saidparticulate ceramic oxide, selected from inorganic phosphates,silicates, borates and aluminates, and a minor effective portion of aliquid phase sintering agent for said ceramic metal oxide;

molding said mixture to form a porous matrix and activating said binderto hold the molded mixture in a porous consolidated form of said matrix;and

heating said porous consolidated form to a temperature above the meltingpoint of said liquid phase sintering agent to thereby cause substantialsintering of said ceramic metal oxide particles.

2. The method of claim 1 wherein said ceramic metal oxide particles aremagnesium oxide.

3. The method of claim 1 wherein said binder is an alkali metalphosphate.

4. The method of claim 3 wherein said liquid phase sintering agent isselected from lithium chloride, lithium bromide, lithium fluoride,lithium iodide, lithium carbonate and lithium sulphate.

5. The method of claim 4 wherein said mixture comprises from about 80 toabout 98 parts by weight of said ceramic metal oxide, from about 1 toabout 10 parts by weight of said binder, and from 1 to about 10 parts byweight of said liquid phase sintering agent.

6. The method of claim 4 wherein said mixture comprises from 90 to 94parts by weight of said ceramic metal oxide, from 2 to 5 parts by weightof said binder, and from 2 to 5 parts by weight of said liquid phasesintering agent.

2. The method of claim 1 wherein said ceramic metal oxide particles aremagnesium oxide.
 3. The method of claim 1 wherein said binder is analkali metal phosphate.
 4. The method of claim 3 wherein said liquidphase sintering agent is selected from lithium chloride, lithiumbromide, lithium fluoride, lithium iodide, lithium carbonate and lithiumsulphate.
 5. The method of claim 4 wherein said mixture comprises fromabout 80 to about 98 parts by weight of said ceramic metal oxide, fromabout 1 to about 10 parts by weight of said binder, and from 1 to about10 parts by weight of said liquid phase sintering agent.
 6. The methodof claim 4 wherein said mixture comprises from 90 to 94 parts by weightof said ceramic metal oxide, from 2 to 5 parts by weight of said binder,and from 2 to 5 parts by weight of said liquid phase sintering agent.